String Reversal Approaches in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of String Reversal Using Array Reverse

What is the output of the following C# code that reverses a string using an array?

DSA C#

string input = "hello";
char[] arr = input.ToCharArray();
Array.Reverse(arr);
string reversed = new string(arr);
Console.WriteLine(reversed);

A olleh

B hello

C Error at Array.Reverse

D olleh\n

💡 Hint

Think about what Array.Reverse does to the character array.

The code converts the string to a char array, reverses it in place, then creates a new string from the reversed array. The output is the reversed string 'olleh'.

❓ Predict Output

intermediate

Output of String Reversal Using StringBuilder

What does this C# code print when reversing a string using StringBuilder?

DSA C#

string input = "world";
var sb = new System.Text.StringBuilder();
for (int i = input.Length - 1; i >= 0; i--) {
    sb.Append(input[i]);
}
Console.WriteLine(sb.ToString());

A Error: StringBuilder.Append invalid

B world

C dlrow

D dlrow\n

💡 Hint

The loop appends characters from the end to the start.

The loop appends characters from the last index to the first, building the reversed string 'dlrow'.

🔧 Debug

advanced

Identify the Error in Recursive String Reversal

What error does this recursive string reversal code produce when run?

DSA C#

string Reverse(string s) {
    if (s.Length == 0) return s;
    return Reverse(s.Substring(1)) + s[0];
}

Console.WriteLine(Reverse(null));

A System.ArgumentNullException

B StackOverflowException

C No error, prints empty string

D System.NullReferenceException

💡 Hint

Consider what happens when s is null and you call s.Length.

Calling s.Length when s is null causes a NullReferenceException.

🧠 Conceptual

advanced

Time Complexity of Different String Reversal Methods

Which option correctly states the time complexity of reversing a string of length n using a for-loop appending characters to a StringBuilder?

A O(n)

B O(n^2)

C O(log n)

D O(1)

💡 Hint

Appending to StringBuilder is efficient; consider how many times the loop runs.

The loop runs n times and appending to StringBuilder is O(1) amortized, so total is O(n).

🚀 Application

expert

Output of In-Place Character Swap String Reversal

What is the output of this C# code that reverses a string by swapping characters in place?

DSA C#

char[] chars = "abcde".ToCharArray();
int left = 0, right = chars.Length - 1;
while (left < right) {
    char temp = chars[left];
    chars[left] = chars[right];
    chars[right] = temp;
    left++;
    right--;
}
Console.WriteLine(new string(chars));

A Error: Cannot assign to chars[left]

B edcba

C abcde

D edcba\n

💡 Hint

Swapping characters from ends moves inward to reverse the array.

The code swaps characters from the start and end moving inward, reversing the array to 'edcba'.

Practice

1. What does the string.Reverse() method in C# do when applied to a string?

easy

A. Returns a sequence of characters in reverse order

B. Modifies the original string to be reversed

C. Throws an error because strings are immutable

D. Converts the string to uppercase

Solution

1. Step 1: Understand string.Reverse() behavior

   The Reverse() method returns an IEnumerable<char> with characters in reverse order but does not change the original string.

2. Step 2: Check string immutability

   Strings in C# are immutable, so Reverse() cannot modify the original string directly.

3. Final Answer:

   Returns a sequence of characters in reverse order -> Option A

4. Quick Check:

   string.Reverse() returns reversed sequence [OK]

Hint: Reverse() returns reversed chars sequence, original string unchanged [OK]

Common Mistakes:

• Thinking Reverse() changes the original string
• Expecting Reverse() to return a string directly
• Confusing Reverse() with ToUpper()

2. Which of the following is the correct syntax to reverse a string str and convert it back to a string in C#?

easy

A. string reversed = new string(str.Reverse().ToArray());

B. string reversed = str.Reverse().ToString();

C. string reversed = string.Reverse(str);

D. string reversed = str.Reverse().Join("");

Solution

1. Step 1: Use Reverse() and convert to string

   str.Reverse() returns IEnumerable<char>, so to get a string, wrap it with new string(...) after converting to array.

2. Step 2: Check each option

   string reversed = new string(str.Reverse().ToArray()); correctly uses new string(str.Reverse().ToArray()). string reversed = str.Reverse().ToString(); calls ToString() on IEnumerable<char>, which returns type name, not reversed string. string reversed = string.Reverse(str); is invalid syntax. string reversed = str.Reverse().Join(""); uses Join incorrectly (should be string.Join).

3. Final Answer:

   string reversed = new string(str.Reverse().ToArray()); -> Option A

4. Quick Check:

   Use new string(str.Reverse().ToArray()) to reverse string [OK]

Hint: Wrap Reverse() with new string() and ToArray() to get reversed string [OK]

Common Mistakes:

• Calling ToString() on Reverse() result
• Using string.Reverse() as static method
• Incorrect use of Join method

3. What will be the output of the following C# code?

string input = "hello";
char[] arr = input.ToCharArray();
Array.Reverse(arr);
string reversed = new string(arr);
Console.WriteLine(reversed);

medium

A. olleh\n

B. hello

C. olleh

D. Error at Array.Reverse

Solution

1. Step 1: Convert string to char array

   input.ToCharArray() creates ['h','e','l','l','o'].

2. Step 2: Reverse the char array

   Array.Reverse(arr) reverses array to ['o','l','l','e','h'].

3. Step 3: Create new string from reversed array

   new string(arr) creates "olleh" string.

4. Step 4: Print reversed string

   Console.WriteLine prints "olleh" without extra characters.

5. Final Answer:

   olleh -> Option C

6. Quick Check:

   Char array reversed then joined = olleh [OK]

Hint: Reverse char array then new string gives reversed string [OK]

Common Mistakes:

• Expecting Array.Reverse to return a value
• Printing char array directly instead of string
• Confusing reversed string with original

4. Identify the error in the following code that attempts to reverse a string:

string s = "world";
string reversed = s.Reverse().ToString();
Console.WriteLine(reversed);

medium

A. s is immutable, so Reverse() cannot be called

B. Console.WriteLine cannot print reversed string

C. Missing semicolon after Reverse()

D. Reverse() returns IEnumerable<char>, ToString() does not convert to string

Solution

1. Step 1: Understand Reverse() return type

   s.Reverse() returns IEnumerable<char>, not a string.

2. Step 2: ToString() on IEnumerable<char>

   Calling ToString() on IEnumerable<char> returns type name, not reversed string.

3. Step 3: Correct way to convert reversed chars to string

   Use new string(s.Reverse().ToArray()) to get reversed string.

4. Final Answer:

   Reverse() returns IEnumerable<char>, ToString() does not convert to string -> Option D

5. Quick Check:

   ToString() on IEnumerable<char> ≠ reversed string [OK]

Hint: ToString() on Reverse() result returns type name, not string [OK]

Common Mistakes:

• Assuming ToString() converts reversed chars to string
• Ignoring need to convert IEnumerable<char> to array
• Thinking Reverse() modifies original string

5. You want to reverse only the vowels in a string "education" while keeping other characters in place. Which approach is best in C#?

hard

A. Use Regex to remove vowels, reverse string, then append vowels at end

B. Convert string to char array, swap vowels using two pointers, then create new string

C. Use string.Reverse() to reverse entire string and then replace consonants

D. Call ToCharArray(), reverse array, then convert back to string

Solution

1. Step 1: Understand problem requirement

   Only vowels should be reversed, consonants stay in place.

2. Step 2: Evaluate approaches

   Convert string to char array, swap vowels using two pointers, then create new string uses two pointers on char array to swap vowels in place, preserving consonants. Use string.Reverse() to reverse entire string and then replace consonants reverses entire string, losing consonant positions. Use Regex to remove vowels, reverse string, then append vowels at end removes vowels, losing original order. Call ToCharArray(), reverse array, then convert back to string reverses entire string, not selective vowels.

3. Step 3: Choose best approach

   Convert string to char array, swap vowels using two pointers, then create new string is efficient and correct for selective vowel reversal.

4. Final Answer:

   Convert string to char array, swap vowels using two pointers, then create new string -> Option B

5. Quick Check:

   Two-pointer vowel swap preserves consonants [OK]

Hint: Use two pointers on char array to reverse vowels only [OK]

Common Mistakes:

• Reversing entire string instead of vowels only
• Removing vowels and losing original order
• Not converting string to char array before swapping